B. Monemar

Infrared dielectric functions and phonon modes of high-quality ZnO films

Infrared dielectric function spectra and phonon modes of high-quality, single crystalline, and highly resistive wurtzite ZnO films were obtained from infrared (300–1200 cm−1) spectroscopic ellipsometry and Raman scattering studies. The ZnO films were deposited by pulsed-laser deposition on c-plane sapphire substrates and investigated by high-resolution x-ray diffraction, high-resolution transmission electron microscopy, and Rutherford backscattering experiments. The crystal structure, phonon modes, and dielectric functions are compared to those obtained from a single-crystal ZnO bulk sample. The film ZnO phonon mode frequencies are highly consistent with those of the bulk material. A small redshift of the longitudinal optical phonon mode frequencies of the ZnO films with respect to the bulk material is observed. This is tentatively assigned to the existence of vacancy point defects within the films. Accurate long-wavelength dielectric constant limits of ZnO are obtained from the infrared ellipsometry anal...

Defect related issues in the “current roll-off” in InGaN based light emitting diodes

Defect related contributions to the reduction of the internal quantum efficiency of InGaN-based multiple quantum well light emitting diodes under high forward bias conditions are discussed. Screening of localization potentials for electrons is an important process to reduce the localization at high injection. The possible role of threading dislocations in inducing a parasitic tunneling current in the device is discussed. Phonon-assisted transport of holes via tunneling at defect sites along dislocations is suggested to be involved, leading to a nonradiative parasitic process enhanced by a local temperature rise at high injection.

Mechanism for low-temperature photoluminescence in GaNAs/GaAs structures grown by molecular-beam epitaxy

The mechanism for low-temperature photoluminescence (PL) emissions in GaNAs epilayers and GaAs/GaNxAs1 - x quantum well (QW) structures grown by molecular-beam epitaxy is studied in detail, employ ...

Mie Resonances, Infrared Emission, and the Band Gap of InN

Mie resonances due to scattering or absorption of light in InN-containing clusters of metallic In may have been erroneously interpreted as the infrared band gap absorption in tens of papers. Here we show by direct thermally detected optical absorption measurements that the true band gap of InN is markedly wider than the currently accepted 0.7 eV. Microcathodoluminescence studies complemented by the imaging of metallic In have shown that bright infrared emission at 0.7-0.8 eV arises in a close vicinity of In inclusions and is likely associated with surface states at the metal/InN interfaces.

Emission properties of a-plane GaN grown by metal-organic chemical-vapor deposition

We report on the emission properties of nonpolar a -plane GaN layers grown on r -plane sapphire. Temperature-, excitation-density-, and polarization-dependent photoluminescences and spatially resol ...

Luminescence in epitaxial GaN : Cd

Investigations on luminescent properties of good‐quality GaN single‐crystalline epitaxial layers at temperatures in the range 1.6–300 K are reported. The high‐energy region (3.27–3.48 eV) at 1.6 K is dominated by different bound exciton transitions. Peaks are observed at 3.469, 3.454, 3.447, 3.400, 3.377, 3.355, and 3.287 eV at 1.6 K in this region; their possible origin is discussed. At higher temperatures (T > 30 K) the free A‐exciton emission is also clearly observed, and thus its position EAx=3.475±0.0005 eV at 1.6 K could be established (neglecting polariton effects) together with a value of 6.4±0.4 meV for the binding energy of an exciton to a neutral donor. The energy region 3.0–3.3 eV is dominated by the previously studied donor‐acceptor pair spectra, which peak at 3.263±0.003 in all our good‐quality samples (undoped and doped). From the assignment of a subsidiary high‐energy peak as free‐to‐bound transitions, a value of 29±6 meV for the donor binding energy is experimentally determined, in good a...

Some optical properties of the AlxGa1−xAs alloys system

Optical transmission data covering the Γ15V–Γ1C absorption edge are presented, together with photoluminescence (PL) results, for AlxGa1−xAs crystals of high purity [n (293 K) <1017 cm−3] for 0<x<0.9. The results indicate that the fundamental absorption edge is domnnated by electron‐hole interaction even in an an alloy. In the direct‐gap region the bound‐exciton ground‐state peak is clearly resolved. In the indirect‐gap region the relaxation of crystal momentum conservation in optical transitions for an alloy apparently has significant effects on the Γ15V–Γ1C absorption edge, and the discrete peak in absorption spectra disappears. From the variation of the absorption coefficient αth at the Γ15V–Γ1C edge with x, we can deduce the corresponding variation of the excitonic binding energy Eex, excitonic effective mass m*, and electron mass mc for the Γ1C minimum. Extrapolated values for these quantities in AlAs are Eex=5 meV, m*=0.060 m0, and mc=0.11 m0 for a value αth = 2.3×104 cm−1. Further, there is a shift ...

Two-dimensional electron gas density in Al1−xInxN/AlN/GaN heterostructures (0.03≤x≤0.23)

Compared to the AlGaN alloy, which can only be grown under tensile strain on GaN, the AlInN alloy is predicted by Vegards law to be lattice-matched (LM) on fully relaxed GaN templates for an indium content of ~17.5%, i.e., it can be grown either tensely or compressively on GaN. The effect of strain on the polarization induced sheet charge density at the Al1-x Inx N/AlN/GaN heterointerfaces is carefully investigated for 6 and 14 nm thick AlInN barriers including a 1 nm thick AlN interlayer. The barrier indium content ranges at 0.03=x=0.23 for 6 nm thick barriers and 0.07=x=0.21 for 14 nm thick barriers. It is found that the two-dimensional electron gas (2DEG) density varies between (3.5±0.1) × 1013 cm-2 and (2.2±0.1) × 1013 cm-2 for 14 nm thick barriers. Finally, a 2DEG density up to (1.7±0.1) × 1013 cm-2 is obtained for a nearly LM AlInN barrier with ~14.5% indium on GaN as thin as 6 nm.

Properties of Zn‐doped VPE‐grown GaN. I. Luminescence data in relation to doping conditions

Experimental results on optical and electrical properties of VPE‐grown GaN doped with Zn under various conditions of growth are presented. The incorporation of Zn into GaN was found to be critically dependent on growth conditions. Four different Zn‐related acceptorlike centers A–D were found to occur, with broad radiative emissions peaking at 2.87 (A), 2.6 (B), 2.2 (C), and 1.8 eV (D). Their broad shape was found to be due to a moderate lattice relaxation upon optical transitions. A linear model for phonon coupling was found adequate, with a principal mode resonant with the optical band [h/ω, varying between 74 (A level) and about 84 meV (C level)] and an additional lower‐energy mode, yielding Franck‐Condon shifts for these centers ΔFC,A =0.25±0.03, ΔFC,B=0.25±0.04, ΔFC,C =0.28±0.04 eV, and ΔFC,D=0.28±0.05 eV. Data for electrical compensation related to the occurrence of these different emissions indicate that the A level could be the ZnGa substitutional acceptor, while the more efficiently compensating B...

Spatially resolved Hall effect measurement in a single semiconductor nanowire

Efficient light-emitting diodes and photovoltaic energy-harvesting devices are expected to play an important role in the continued efforts towards sustainable global power consumption. Semiconductor nanowires are promising candidates as the active components of both light-emitting diodes and photovoltaic cells, primarily due to the added freedom in device design offered by the nanowire geometry. However, for nanowire-based components to move past the proof-of-concept stage and be implemented in production-grade devices, it is necessary to precisely quantify and control fundamental material properties such as doping and carrier mobility. Unfortunately, the nanoscale geometry that makes nanowires interesting for applications also makes them inherently difficult to characterize. Here, we report a method to carry out Hall measurements on single core-shell nanowires. Our technique allows spatially resolved and quantitative determination of the carrier concentration and mobility of the nanowire shell. As Hall measurements have previously been completely unavailable for nanowires, the experimental platform presented here should facilitate the implementation of nanowires in advanced practical devices.